Semiconductor chips commonly are formed by processing a relatively large flat body of semiconductor material to form numerous regions, each including the electronic circuitry to be incorporated in a single chip, and then cutting the wafer along saw lanes to sever the wafer and thus form individual chips. Each chip typically is a flat rectangular body with generally planar front and rear surfaces and with small edges extending between the front and rear surfaces at the boundaries of these surfaces. Each chip typically has contacts exposed at the front surface which are electrically connected to the circuitry within the chip.
The individual chips commonly are provided in packages which facilitate handling of the chip and mounting of the chip to an external substrate, such as a circuit board or other printed circuit panel. Such packages commonly include a dielectric structure and electrically conductive terminals carried on the dielectric structure, the terminals being electrically connected to the contacts of the chip. In a package of the type referred to a chip scale package, the package occupies an area on the circuit panel only slightly larger than, or the same size as, the area of the chip front surface itself.
As disclosed, for example, in U.S. Pat. No. 5,679,977, the disclosure of which is hereby incorporated by reference herein, the terminals may be movable with respect to the chip. In certain embodiments, the package may incorporate a compliant layer overlying a surface of the chip and carrying the terminals. Movability of the terminals with respect to the chip can compensate for differential thermal expansion between the chip and the circuit panel during manufacture, during service, or both. Also, movability of the terminals with respect to the chip can facilitate engagement of the packaged chip with a testing device. During such engagement, the individual terminals may move in the direction towards or away from the front or rear surface of the chip, commonly referred to as the vertical or “Z” direction. Movement in this direction facilitates engagement of all of the terminals with all of the contacts on the testing device, even if the terminals are not precisely coplanar with one another.
The terminals of a chip package may be in the form of generally flat pads. Small solder balls may be deposited on these pads so that the package may be bonded to a circuit panel by aligning the solder balls with corresponding contact pads of a circuit panel and melting the solder balls in a conventional operation of the type commonly used for surface mounting of components to circuit boards.
As disclosed in U.S. Patent Publication Nos. 2005/0181544; 2005/0181655; and 2005/0173805, the disclosures of which are hereby incorporated by reference herein, as well as in U.S. Pat. No. 6,774,317, the disclosure of which is also incorporated by reference herein, a chip or other microelectronic element may be provided with terminals in the form of posts, and such posts may be bonded to a circuit panel in a similar soldering operation. In certain embodiments, the posts can provide particularly good engagement with test fixtures during testing before mounting to the circuit panel.
Semiconductor chip packages most commonly have been made by assembling individual chips with the other elements constituting the package. This requires handling and placement of the “bare” or unpackaged semiconductor chips. Various proposals have been advanced for making chip packages in a wafer-scale operation, as by uniting the wafer with the other elements of the packaged chip before severing the wafer to form individual chips. For example, the aforementioned '977 patent discloses certain embodiments of processes which form the chip packages in this manner.
Some processes for making packaged chips on a wafer scale have suffered from certain drawbacks. Where a compliant layer is formed on the front surface of a wafer, as, for example, by depositing a curable material and then curing the material to form the layer, the compliant layer tends to cause warpage of the wafer. Such warpage may occur, for example, due to differential expansion and contraction of the compliant material and the semiconductor material constituting the wafer during curing of the compliant material or during other processing operations. Such warpage makes it difficult to perform other processing operations such as forming the terminals and the connections between the terminals and the contacts. While such warpage can be reduced by reducing the thickness of the compliant layer, a thin compliant layer may not provide sufficient movability to the terminals.
As disclosed, for example, in U.S. Pat. No. 6,847,101, the disclosure of which is hereby incorporated by reference herein, a compliant layer may include individual elements in the form of protrusions projecting from the surface of the chip or wafer, with the terminals being disposed at the tops of such protrusions and with electrical connections between the terminals and the contacts including metallic strips extending downwardly from the tops of such protrusions towards the front surface of the chip or wafer. Such individual bumps or protrusions can provide significant compliance without the drawbacks associated with a continuous layer. However, the process for forming the terminals and connections is somewhat more demanding than the process for forming terminals and connections on a generally planar surface provided by a continuous compliant layer.
Proposals have been advanced for increasing the compliance of compliant layers or individual protrusions by forming the protrusions or layer with hollow cavities beneath the terminals. Such hollow cavities allow displacement of the terminals in the Z-direction toward the chip without the need to compress a solid body of compliant material disposed between the terminal and the chip surface. However, certain structures formed with such hollow cavities can be unreliable under some circumstances. Although the present invention is not limited by any theory of operation, it is believed that such reliability problems result at least in part from changes in the pressure of the gas trapped within such cavities.
Other designs have used compliant materials in the form of open-celled foams to provide substantial compressibility and thus facilitate Z-direction movement of the terminals. Forming compliant layers or protrusions from foam tends to create difficulties with contamination. Materials such as plating solutions or etchants used to form the terminals after deposition of the foam can infiltrate into the foam and attack the structure of the compliant layer or the chip itself during service.
Accordingly, despite the considerable effort in the art heretofore devoted to development of compliant packages and methods of forming the same, further improvements would be desirable.